Electrochemical glucose test strips, such as those used in the OneTouch® Ultra® whole blood testing kit, which is available from LifeScan, Inc., are designed to measure the concentration of glucose in a blood sample from patients with diabetes. The measurement of glucose is based upon the specific oxidation of glucose by the flavo-enzyme glucose oxidase. The reactions which may occur in a glucose test strip are summarized below in Equations 1 and 2.D-Glucose+GO(ox)→Gluconic Acid+GO(red)  (1)GO(red)+2Fe(CN)63−→GO(ox)+2Fe(CN)64−  (2)
As shown in Equation 1, glucose is oxidized to gluconic acid by the oxidized form of glucose oxidase (GO(ox). It should be noted that GO(ox) may also be referred to as an “oxidized enzyme”. During the reaction in Equation 1, the oxidized enzyme GO(ox) is converted to its reduced state which is denoted as GO(red) (i.e., “reduced enzyme”). Next, the reduced enzyme GO(red) is re-oxidized back to GO(ox) by reaction with Fe(CN)63− (referred to as either the oxidized mediator or ferricyanide) as shown in Equation 2. During the re-generation of GO(ox) back to its oxidized state GO(CN)63− is reduced to Fe(CN)64− (referred to as either reduced mediator or ferrocyanide).
When the reactions set forth above are conducted with a test voltage applied between two electrodes, a test current may be created by the electrochemical re-oxidation of the reduced mediator at the electrode surface. Thus, since, in an ideal environment, the amount of ferrocyanide created during the chemical reaction described above is directly proportional to the amount of glucose in the sample positioned between the electrodes, the test current generated would be proportional to the glucose content of the sample. A mediator, such as ferricyanide, is a compound that accepts electrons from an enzyme such as glucose oxidase and then donates the electrons to an electrode. As the concentration of glucose in the sample increases, the amount of reduced mediator formed also increases, hence, there is a direct relationship between the test current resulting from the re-oxidation of reduced mediator and glucose concentration. In particular, the transfer of electrons across the electrical interface results in a flow of test current (2 moles of electrons for every mole of glucose that is oxidized). The test current resulting from the introduction of glucose may, therefore, be referred to as a glucose current.
Because it can be very important to know the concentration of glucose in blood, particularly in people with diabetes, test meters have been developed using the principals set forth above to enable the average person to sample and test their blood to determine the glucose concentration at any given time. The glucose current generated is monitored by the test meter and converted into a reading of glucose concentration using an algorithm that relates the test current to a glucose concentration via a simple mathematical formula. In general, the test meters work in conjunction with a disposable test strip that includes a sample receiving chamber and at least two electrodes disposed within the sample receiving chamber in addition to the enzyme (e.g. glucose oxidase) and the mediator (e.g. ferricyanide). In use, the user pricks their finger or other convenient site to induce bleeding and introduces a blood sample to the sample receiving chamber, thus starting the chemical reaction set forth above.
In electrochemical terms, the function of the meter is two fold. Firstly, it provides a polarizing voltage (approximately 0.4 V in the case of OneTouch® Ultra®) that polarizes the electrical interface and allows current flow at the carbon working electrode surface. Secondly, it measures the current that flows in the external circuit between the anode (working electrode) and the cathode (reference electrode). The test meter may, therefore be considered to be a simple electrochemical system that operates in a two-electrode mode although, in practice, third and, even fourth electrodes may be used to facilitate the measurement of glucose and/or perform other functions in the meter.
In most situations, the equation set forth above is considered to be a sufficient approximation of the chemical reaction taking place on the test strip and the test meter outputting a sufficiently accurate representation of the glucose content of the blood sample. However, under certain circumstances and for certain purposes, it may be advantageous to improve the accuracy of the measurement. For example, blood samples having a high hematocrit level or low hematocrit level may cause a glucose measurement to be inaccurate.
A hematocrit level represents a percentage of the volume of a whole blood sample occupied by red blood cells. The hematocrit level may also be represented as a fraction of red blood cells present in a whole blood sample. In general, a high hematocrit blood sample is more viscous (up to about 10 centipoise at 70% hematocrit) than a low hematocrit blood sample (about 3 centipoise at 20% hematocrit). In addition, a high hematocrit blood sample has a higher oxygen content than low hematocrit blood because of the concomitant increase in hemoglobin, which is a carrier for oxygen. Thus, the hematocrit level can influence the viscosity and oxygen content of blood. As will be later described, both viscosity and oxygen content may change the magnitude of the glucose current and in turn cause the glucose concentration to be inaccurate.
A high viscosity sample (i.e., high hematocrit blood sample) can cause the test current to decrease for a variety of factors such as a decrease in 1) the dissolution rate of enzyme and/or mediator, 2) the enzyme reaction rate, and 3) the diffusion of a reduced mediator towards the working electrode. A decrease in current that is not based on a decrease in glucose concentration can potentially cause an inaccurate glucose concentration to be measured.
A slower dissolution rate of the reagent layer can slow down the enzymatic reaction as shown in Equations 1 and 2 because the oxidized enzyme GO(ox) must dissolve first before it can react with glucose. Similarly, ferricyanide (Fe(CN)63−) must dissolve first before it can react with reduced enzyme GO(red). If the undissolved oxidized enzyme GO(ox) cannot oxidize glucose, then the reduced enzyme GO(red) cannot produce the reduced mediator Fe(CN)64− needed to generate the test current. Further, oxidized enzyme GO(ox) will react with glucose and oxidized mediator Fe(CN)63− more slowly if it is in a high viscosity sample as opposed to a low viscosity sample. The slower reaction rate with high viscosity samples is ascribed to an overall decrease in mass diffusion. Both oxidized enzyme GO(ox) and glucose must collide and interact together for the reaction to occur as shown in Equation 1. The ability of oxidized enzyme GO(ox) glucose to collide and interact together is slowed down when they are in a viscous sample. Yet further, reduced mediator Fe(CN)64− will diffuse to the working electrode slower when dissolved in a high viscosity sample. Because the test current is typically limited by the diffusion of reduced mediator Fe(CN)64− to the working electrode, a high viscosity sample will also attenuate the test current. In summary, there are several factors that cause the test current to decrease when the sample has an increased viscosity.
A high oxygen content may also cause a decrease in the test current. The reduced enzyme (GO(red)) can reduce oxygen (O2) to hydrogen peroxide as shown be Equation 3.GO(red)+O2→GO(ox)+H2O2  (3)
As noted earlier, the reduced enzyme GO(red) can also reduce ferricyanide (Fe(CN)63−) to ferrocyanide (Fe(CN)64−) as shown in Equation 2. Thus, oxygen can compete with ferricyanide for reacting with the reduced enzyme (GO(red)). In other words, the occurrence of the reaction in Equation 3 will likely cause a decrease in the rate of the reaction in Equation 2. Because of such a competition between ferricyanide and oxygen, a higher oxygen content will cause less ferrocyanide to be produced. In turn, a decrease in ferrocyanide would cause a decrease in the magnitude of the test current. Therefore, a high oxygen content blood sample can potentially decrease the test current and affect the accuracy of the glucose measurement.
As such, applicants have great interest in the development of methods reducing the effects of hematocrit on a glucose measurement. In certain protocols, a pre-cast blood filtering membrane that is separate from the reagent layer has been employed to remove red blood cells and thereby reduce the hematocrit effect. The pre-cast blood filtering membrane which is separated from the reagent layer can be disposed on the working electrode. The use of a discrete pre-cast blood filtering membrane is unsatisfactory in that it requires a more complex test strip, increased sample volume, and increased testing time. The blood filtering membrane retains a certain amount of blood that does not contact the working electrodes causing a need for a larger blood sample. In addition, a finite amount of time is needed for the blood to be filtered by the membrane causing an increase in the overall test times. Thus, applicants recognize that it would be advantageous to reduce the effects of hematocrit without using a pre-cast blood filtering membrane that is separate from the reagent layer.
In the prior art, the hematocrit effect may be reduced by applying multiple test voltages such as, for example, a sinusoidal test voltage. However, applying a sinusoidal test voltage results in a more complex and expensive test meter. Further, the test meter needs to measure the test currents accurately and precisely at pre-determined time intervals. The electronic components can be expensive and complicated for a test meter to accurately and precisely apply multiple test voltages.
Applicants realize that it would be advantageous to implement a system having a test meter that applies only one test voltage and a test strip that does not use a pre-cast membrane to reduce the effects of hematocrit. The system instead uses a test strip having a working electrode with a plurality of microelectrodes formed thereon. More particularly, applicants recognizes that it would be advantageous to develop an algorithm that mathematically processes the collected test current using one test voltage such that an accurate glucose concentration can be determined that reduces the effects of hematocrit.
Furthermore, applicants have determined that it would be beneficial to provide a mechanism whereby the test meter can differentiate between a bodily fluid, for example whole blood, and a control solution. Similarly, it would be beneficial to provide a method whereby a test meter can determine if a test strip includes a plurality of microelectrodes formed on a working electrode.